This invention relates generally to methods for preparing metal surfaces for soldering, and more particularly concerns precleaner compositions, soldering flux compositions, and methods for use of such compositions.
During the manufacture of electronic components, including printed wire assemblies and boards and especially integrated circuits, those components are subjected to elevated temperatures which accelerate the formation of oxides on the surface of the metal leads. These oxides and other contaminants must be removed from the leads before the leads can be soldered. Typically, integrated circuit leads are precleaned, fluxed, and then tinned with solder so that the leads are protected against further oxidation and can be easily soldered or reflowed later.
Generally the formulations for both precleaners and soldering fluxes include an activator system of one or more activators, a surfactant, and a solvent carrier. An activator is the ingredient which directly reacts with the oxides and contaminants on the metal leads to remove such oxides and contaminants. The surfactant is used to insure that the surface tension of the precleaner and the flux is such that they will easily wet the oxides and contaminants so that the activators are in intimate contact with the oxides and contaminants thereby enhancing the chemical action of the activators. The surfactants may conventionally include any of a large number of ionic and non-ionic surfactants. The solvent carriers for precleaners may conventionally include water, short-chained alcohols, glycol ethers, and the like. Solvent carriers for fluxes may include short-chained alcohol, glycol ethers, water, aliphatic and aromatic solvents, and the like.
Conventional precleaner activators include, either singly or in combination, mineral acids and/or salts and/or bases. The precleaner acids may include hydrochloric, hydrobromic, hydrofluoric, sulfuric, nitric, phosphoric, among others. The precleaner salts may include ammonium chloride, stannus chloride, ammonium persulphate, alkylaminehydrochlorides, zinc chloride, zinc ammonium chloride, alkanolamine hydrochlorides, among others. The precleaner bases may include mono-, di-, and triethanolamine, sodium hydroxide, potassium hydroxide, ammonium hydroxide, among others. The flux activators generally include a large assortment of organic and inorganic acids including halide-containing acids.
Halide-containing precleaners and fluxes have been found effective in removing oxides and contaminants on metal leads resulting from high temperature processing. The halides in the precleaners and fluxes, however, attack the leaded glass frit of the integrated circuits and cause corrosion and degradation of the components if the halide migrates into the component through flaws in the surface. As a result, halide-free precleaners and fluxes have been developed to preclean and flux integrated circuit leads in order to avoid the problems created by the halides in conventional precleaners and fluxes.
Halide-free mineral acids such as nitric acid and sulfuric acid have often been used in high concentrations in precleaner formulations but those acids may also lead to component malfunction. Nitric acid and sulfuric acid are known to etch and passivate the commonly used nickel alloy of the integrated circuit leads so that the leads cannot thereafter be successfully wetted by solder. Furthermore, nitric acid and sulfuric acid when used in precleaners at conventional high concentrations and temperatures represent a considerable health hazard due to their high degree of corrosivity and their evolution of toxic nitrogen oxides.
Halide-free fluxes which generally contain organic acids such as levulenic, citric, tartaric, adipic, acetic, and the like, have been used after the precleaners to complete the metal preparation prior to the application of solder to the metal leads. These formulations of halide-free fluxes may also contain organic amines and/or organic salts. Though safer for the components, these preparations generally lack the necessary activity to clean heavily oxidized component leads.
Laudenslager U.S. Pat. No. 3,199,190 discloses the use of hydroxyl containing organic acids such as hydroxyacetic acid in conjunction with an alkanolamine, such as monoethanolamine. Alkanolamines such as monoethanolamine, however, attack the leaded glass frit and produce azides which are left as residue.
Stright U.S. Pat. No. 2,581,820 discloses a combination of an organic acid and an ureas in conjunction with conventional wetting agents and solvent carrier. The organic acid is preferably lactic acid, and urea is the preferred ureas. The resulting flux, however, leaves a persistent residue which must be subsequently cleaned.
Jordan et al U.S. Pat. No. 3,814,638 discloses the use of a chelating agent such as 8-hydroxyquinoline which inactivates the metallic ions once they have been removed from the leads during the fluxing operation and prevents those metal ions from causing further oxidation of the metal leads during the fluxing operation.